The hydraulic system for a vehicle transmission serves important lubrication, cooling, and clutch actuation functions necessary for operation of the transmission. Generally, hydraulic pressure and flow within the hydraulic system is provided by an engine-driven hydraulic pump having a rotational speed proportional to engine speed. The hydraulic pump must be designed to meet peak demands, such as hydraulic fluid demand during shifting of the transmission, hot engine idle speed garage shifts, and, in certain transmissions, a hydraulic park mode in an electronic transmission range selector (ETRS) system. Additionally, hydraulic requirements of the transmission may still exist when the engine (and thus the engine-driven pump) is off, such as in an electric-only mode in a hybrid electromechanical powertrain. Pump torque and horsepower requirements rise with increasing pump speed, and have an associated negative effect on fuel economy; thus, it is desirable to limit the flow delivery of the pump, to the extent possible. On-demand pumps, which may include binary pumps, are operable at a full or a partial volumetric output. Thus, such pumps may be operated at a fuel-saving partial output whenever possible, operating at full output only when necessary.
Finally, packaging space may be very limited in certain vehicle designs, driving the location of and potentially the size and type of pumps that may be used. For instance, an “on-axis” hydraulic pump has a rotational axis that is aligned (i.e., coaxial) with an axis of rotation of an input member of the transmission (i.e., the transmission axis). An on-axis pump may extend the overall length of the transmission. An “off-axis” pump has an axis of rotation that is offset from and not concentric or coaxial with the transmission axis. Off-axis pumps offer increased flexibility in packaging location. However, in a rear wheel-drive vehicle, if the transmission “tunnel” running fore and aft in the vehicle (i.e., the packaging space allotted to the transmission, typically tunnel-shaped and under the vehicle floor) has relatively little free packaging space, packaging of an off-axis pump appropriately sized to handle the hydraulic requirements of the transmission may be difficult or impossible.